The present invention relates to a dome made of aluminum alloy, particularly intended to form the bottom of a tank, and to the method of manufacturing it.
It is known that rockets and space shuttles comprise large-sized tanks for storing the fuels intended to propel them. The bottoms of such tanks are made of aluminum alloy and have the shape of a dome several meters in diameter.
To manufacture a tank bottom such as this, it is commonplace for a number of curved sectors, for example eight curved sectors, to be produced individually, these then being secured together via radial weld lines to form said tank bottom.
Such a method of manufacture is lengthy and expensive, particularly because of the high number of constituent parts and the long lengths of the numerous weld lines. Hence, the main object of the present invention is to reduce the cost of manufacture of such tank bottoms.
To this end, according to the invention, the dome made of a weldable aluminum alloy, particularly intended to form the bottom of a tank, is noteworthy in that it consists of a dish of rounded shape with an opening at its top, and of a cap welded onto said dish to seal off its opening.
Thus, the dome according to the present invention is formed with just two partsxe2x80x94the dish and the cap alongxe2x80x94a single weld line of relatively limited length. This then results in a significant reduction in the cost of manufacture by comparison with the usual method recalled herein above.
Of course, said cap may easily be manufactured by any known forming operation. As regards the manufacture of the dish, the applicant company has developed a particularly advantageous method.
Specifically, according to the invention, the method for producing a dish of rounded shape, open at its top, of a weldable aluminum alloy, particularly for producing the bottom of a tank, is notable in that:
a) a mold is produced, the interior face of the cavity of which corresponds to the shape of said dish, said mold being closed at its top and open at the opposite end from said top;
b) a blank in the shape of a cone frustum is produced, by bending and welding, from at least one flat piece in the shape of a portion of an annulus of such an untempered aluminum alloy;
c) said frustoconical blank is completely inserted in said mold, so that it adopts an initial position in which it is concentric with said mold, with:
its exterior frustoconical face facing said interior face of said cavity;
its small base resting on said interior face of the cavity near the top of said mold; and
its large base close to said interior face of the cavity near the opening of said mold;
d) a deformable pressing wall is arranged peripherally inside said frustoconical blank, against its interior face and near its small base;
e) said frustoconical blank and said deformable pressing wall are subjected to a first phase of a heat treatment with pressure, during which phase said frustoconical blank and said deformable pressing wall in contact therewith bend toward the interior face of said cavity at the same time progressing toward the top of said mold, being swallowed by the latter;
f) the progression of said frustoconical blank and of said deformable pressing wall toward the top of said mold is halted when said blank and said pressing wall reach a predetermined position of swallowing;
g) in this predetermined position of swallowing, said heat treatment with pressure is continued by a second phase, during which phase the exterior face of said blank is molded to the interior face of said cavity, with said pressing wall pressed against the interior face of said blank;
h) at the end of said heat treatment with pressure, said blank shaped into a rounded dish, open at the top, is extracted from the mold.
The applicant company has found that:
due to the progressive swallowing of the frustoconical blank by the mold, under the action of heat and pressure, it is possible to limit to an acceptable level the mechanical stresses experienced by said blank for making it change from the frustoconical shape to the shape of a rounded dish, for example a spherical dish;
halting the swallowing of said frustoconical blank at a predetermined value avoids an undesirable build-up of material of the frustoconical blank near the top of the mold; and
the action of the pressing wall near the small base of the blank, in collaboration with the halting of the swallowing, makes it possible for said dish to be given a perfect shape near its opening, avoiding buckling of said dish at this point.
Of course, it is possible to contrive for said dish to have a thickness which is constant at all points or on the contrary, to have a greater thickness in some of its regions, by adjusting accordingly the distribution of thickness of said flat piece or pieces in the shape of (a) portion(s) of an annulus, from which the frustoconical blank is made.
In an advantageous embodiment of the method according to the invention, after said second phase, said heat treatment with pressure is continued by a third phase of tempering, allowing said dish open at its top to be structurally hardened.
Said anti-buckling deformable pressing wall may be formed of a set of deformable leaves, preferably curved, distributed around the periphery of the interior face of said frustoconical blank. It is then advantageous for said deformable leaves to be secured to one another at their ends near said small base of the frustoconical blank, for example by means of a peripheral strip.
The deformation pressure to which said frustoconical blank and said pressing wall are subjected during the heat treatment is preferably exerted by a pressing bladder introduced into said frustoconical blank. The wall of said mold is then advantageously pierced with through holes through which a vacuum can be pulled between said bladder and the interior face of the mold cavity so as to press said bladder against said frustoconical blank and said pressing wall so as to deform these until they are pressed firmly against the interior face of said mold cavity.
In order to allow controlled swallowing of said frustoconical blank and of said pressing wall by the mold during the first phase of said heat treatment, and for these to be halted in said predetermined position of swallowing, a sliding connection with end-of-travel stop is preferably provided between the edge of the opening of the mold and the large base of said frustoconical blank. For this purpose, the large base of the frustoconical blank may bear an interior protruding rim which forms part of said end-of-travel.